Gas atomization is a commonly used technique for economically making fine metallic powder by melting the metallic material and then impinging a gas stream on the melt to atomize it into fine molten droplets that are solidified to form the powder. One particular gas atomization process is described in the Ayers and Anderson U.S. Pat. No. 4,619,845 wherein a molten stream is atomized by a supersonic carrier gas to yield fine metallic powder (e.g., powder sizes of 10 microns or less).
The metallic powder produced by gas atomization processes is suitable for fabrication into desired end-use shapes by various powder consolidation techniques. However, as a result of the fine size of gas atomized powder (i.e., powder having a high surface to volume ratio), the metallic powder is more susceptible to environmental degradation, such as oxidation, corrosion, contamination, etc. than the same metallic material in bulk form. Some alloy powders, in particular aluminum and magnesium, have been made more stable to environmental constituents by producing a thin oxide film on the powder particles during or after gas atomization. Production of stabilizing refractory films during gas atomization has been effected on aluminum powder by utilizing a recycled gas mixture (flue gas) for the atomization gas and ambient air for the spray chamber environment. During the atomization process the oxygen (or other reactive gas species, like carbon) in this complex gas environment reacts with the aluminum to form a coating on the particles. Stabilizing carbonate/oxide films have been produced on reactive ultrafine metal powders, such as carbonyl-processed iron, following their initial formation by slowly bleeding carbon dioxide gas into the formation chamber and allowing a long exposure time before removal of the particulate. Slow bleeding rates are required to prevent such a temperature rise of the powder during initial reaction as could cause rapid catastrophic powder burning or explosion.
The problem of environmental degradation is especially aggravated when the metallic material includes one or more highly reactive alloying elements that are prone to chemically react with constituents of the environment such as oxygen, nitrogen, carbon, water in the vapor or liquid form and the like. The rare earth-iron-boron alloys (e.g., Nd--Fe--B alloys) developed for magnetic applications represent a particularly troublesome alloy system in terms of reactivity to environmental constituents of the type described, even to the extent of exhibiting pyrophoric behavior in the ambient environment. There is a need to protect such atomized reactive alloy powders from environmental degradation during fabrication operations to form magnet shapes and during use of the magnet in its intended service environment where the magnet is subjected to the environmental constituents described above.
Rare earth-iron-boron alloy powders (made from mechanically milled rapidly solidified ribbon) have been fabricated into magnet shapes by compression molding techniques wherein the alloy powder is mixed at elevated temperature, such as 392.degree. F., with a suitable resin or polymer, such as polyethylene and polypropylene, and the mixture is compression molded to a magnet shape of simple geometry. A surfactant chemical is blended with the resin or polymer prior to mixing with the alloy powder so as to provide adequate wetting and rheological properties for the compression molding operation. Elimination of the need for surfactant chemical is desirable as a way to simplify fabrication of the desired magnet shape and to reduce the cost of fabricating magnets from such powder alloys.
It is an object of the present invention to provide a method of making metallic powder from a melt having a composition including one or more reactive alloying elements in selected concentration to provide desired end-use properties (e.g., magnetic properties) wherein a beneficial coating or layer is formed in-situ thereon that protects the reactive powder against environmental (oxidation, corrosion, etc.) attack.
It is another embodiment of the invention to provide a method of making metallic powder from a melt of the type described in the preceding paragraph wherein a beneficial coating or layer is formed on the powder to facilitate subsequent fabrication of the powder to end-use shapes by mixing with a polymeric or other binder.
It is another object of the present invention to provide reactive metallic powder having one or more coatings that protect against environmental degradation during fabrication of the powder to end-use shapes and during use in the intended service environment.
It is another object of the invention to provide a method of making such coated powder in a manner controlled to avoid altering the powder composition to an extent that would degrade the powder end-use properties (e.g., magnetic properties).